In directional or controlled trajectory drilling, the vertical inclination and azimuth of a drilled bore may be controlled such that the bore may extend from the surface to a target area which is not vertically aligned with the point on the surface where drilling commences. This permits a wide area to be accessed from a single drilling location and is therefore particularly useful in offshore drilling operations.
Conventionally, rotation of the drill bit mounted on the lower end of the drill string is achieved by rotation of the entire drill string, by a rotating turntable or “top drive” on the surface, and often also by a downhole motor located on the drill string adjacent the bit. The downhole motor is usually driven by the drilling fluid which is pumped through the string. Steerable downhole motors include a “bent” housing or elbow which introduces a small deviation (around 1°) in the end portion of the drill string. When the entire string is rotating such an elbow has little or no effect on the bore trajectory. However, if the string is stopped and then adjusted such that the motor bend is in a desired direction, rotating the drill bit using only the downhole motor will result in the trajectory of the well deviating. However, progress when drilling in this manner, without rotation of the drill string, tends to be relatively slow.
Various attempts have been made to provide drilling apparatus which will permit bore trajectory to be varied or controlled while still rotating the drill string, in some instances by providing a non-rotating eccentric mass on the drill string adjacent the drill bit. In some proposals, the mass engages the “low” portion of the bore wall and supports the drill string. A radially extending blade is mounted on the mass and engages the bore to produce a lateral force on the drill string causing the drill bit to deviate from its existing path, or at least prevents further deviation in the direction of the blade. However, the success of such apparatus has been limited as the mass provides an unstable support for the heavy drill string, such that the mass is likely to topple and be moved to one side by the string, which will tend to move downwards to occupy the lower part of the bore. Examples of such arrangements are illustrated in U.S. Pat. Nos. 4,638,873 and 4,220,213.
WO96\31679 describes a surface controlled well bore directional steering tool comprising a mandrel for forming part of the drill string, and two eccentric sleeves. The outer sleeve has an eccentric bore that forms a pregnant or weighted side. Two stabiliser shoes are provided on either side of the sleeve at 90° to the pregnant housing. The inner sleeve has a further eccentric longitudinal bore that contains the mandrel. The relative orientations of the sleeves may be controlled to move the mandrel to one side of the pregnant housing, thus transmitting a fulcrum force to the bit. An electric motor in the housing may be activated from surface to rotate the inner sleeve. As far as the present applicant is aware, this tool has not been commercialised, and it is believed that the tool would prove difficult to operate.
Applicant's GB 2,343,470 and U.S. patent application Ser. No. 09/435,453, and also WO97\47848 and U.S. patent application Ser. No. 09/202,342, the disclosures of which are incorporated herein by reference, describe arrangements including non-rotating offset masses provided in combination with pairs of stabilisers at either end of the mass, one of the stabilisers being adjustable relative to the mass to provide a desired offset of the drill string in the bore.
Other forms of directional drilling apparatus for controlling hole direction or inclination by providing eccentric or offset blades or members are described in U.S. Pat. Nos. 3,062,303, 3,092,188, 3,650,338, 3,825,081 and 4,305,474. U.S. Pat. No. 6,216,802 describes an arrangement for orienting a drilling assemble featuring two drive shafts coupled by a universal joint, with an orientation collar operable to change the orientation of one of the drive shafts.
It is among the objectives of the embodiments of the present invention to provide improved directional drilling apparatus utilising an offset or eccentric mass, or by offsetting the drill string itself.